This invention relates to a process for producing nanoscale polymeric fibers having morphologies and textures, especially having open porous structures, and also their modification and use.
Owing to their high surface/volume ratio and their differences to typical ordering structures in macroscopic systems, nanoscale materials have special physical and chemical properties, described for example in Gleitner, H.; xe2x80x9cNanostructured Materialsxe2x80x9d, in Encyclopedia of Physical Science and Technology, Vol. 10, p. 561 ff. These include short-range magnetic properties in the case of metallic or oxidic materials, easy field-induced tunneling of electrons from filament tips, or particularly advantageous biocompatibilities due to nanoscale microdomains. These differences in property profiles compared with macroscopic materials have led to technological innovations in microelectronics, display technology, surface technology, catalyst manufacture and medical technology, especially as carrier materials for cell and tissue cultures.
Fiber materials having filament diameters of less than 300 nm, in fact down to a few 10 nm, are useful, if electroconductive, as field electron emission electrodes according to WO 98/1588. They similarly offer technological benefits in semiconductor systems as described in U.S. Pat. No. 5,627,140 and also as catalyst systems having improved activity profiles, described in WO 98/26871. Such fibers can be chemically modified and be provided with chemical functions, for example by chemical etching or by plasma treatment, processed into woven fabrics or compacted into feltlike materials. They can be incorporated, not only in unorganized form but also in an aligned or organized form as wovens, drawn-loop knits, formed-loop knits or in some other compacted arrangement, into macroscopic construction material systems in order that mechanical or other physical properties of the materials of construction may be improved.
According to WO 00/22207, fibers having diameters of less than 3000 nm can be produced using compressed gases expanding from specific nozzles. Prior art also includes electrostatic spinning processes described in DE 100 23 456.9. GB 2 142 870, for example, describes an electrostatic spinning process for manufacturing vascular grafts.
Nanofibers can be used as templates for coatings applied to the fibers from solutions or by vapor deposition for example. This makes it possible to deposit on the fibers not only polymeric, ceramic, or oxidic or glassy materials but also metallic materials in the form of uninterrupted layers. By dissolving, vaporizing, melting or pyrolyzing the inner, polymeric template fiber it is thus possible to obtain tubes in a wide variety of materials of construction whose inner diameter can be varied from 10 nm up to a few xcexcm, depending on the filament diameter, and whose wall thicknesses are in the nm or xcexcm range, depending on coating conditions. The production of such nano- or mesotubes is described in DE 10 23 456.9.
For certain applications of nanoscale fibers it appears to be advantageous to create a large surface area using porous materials. In WO 97/43473, fibers are provided with a porous coating. A subsequent pyrolysis treatment provides high-porosity fibers that are advantageous for catalytic uses for example.
The above-described processes for producing porous nano- and mesoscale fibers require plural steps and are time and cost intensive. Furthermore, porous fiber materials offer additional technical benefits over uninterrupted, solid fibers, since they have a substantially larger surface area. True, nanotubes have a very large surface area, but are very inconvenient to produce because of the pyrolysis step.
EP 0 047 795 describes polymeric fibers having a solid core and a porous, foamy sheath surrounding the core. The fiber core is said to possess high mechanical stability, while the porous sheath has a large surface area. Yet in the case of very surface-active applications, for example filtrations, the porous structure created according to EP 0 047 795 is frequently inadequate.
It is an object of the present invention to provide nano- and mesoscale polymeric fibers having a very large surface area using a simple process.
This object is achieved by porous fiber comprising a polymeric material, the fiber having a diameter of 20 to 4000 nm and pores in the form of channels extending at least to the core of the fiber and/or through the fiber.
The invention further provides a process for producing porous fiber from a polymeric material, which comprises electrospinning a 3 to 20% by weight solution of a polymer in a volatile organic solvent or solvent mixture using an electric field above 105 V/m to obtain a fiber having a diameter of 20 to 4000 nm and pores in the form of channels extending at least to the core of the fiber and/or through the fiber.
Electrospinning processes are described for example in Fong, H.; Reneker, D. H.; J. Polym. Sci., Part B, 37 (1999), 3488, and in DE 100 23 456.9.
Field strengths vary from 20 to 50 kV, preferably from 30 to 50 kV, and linear spinning speeds (exit speed at spinneret) from 5 to 20 m/s, preferably from 0.8 to 15 m/s.
Porous fiber structures according to the invention comprise polymer blends or copolymers, preferably polymers such as polyethylene, polypropylene, polystyrene, polysulfone, polylactides, polycarbonate, polyvinylcarbazole, polyurethanes, polymethacrylates, PVC, polyamides, polyacrylates, polyvinylpyrrolidones, polyethylene oxide, polypropylene oxide, polysaccharides and/or soluble cellulose polymers, for example cellulose acetate.
These polymers may be used individually or in the form of their blends. In a particular embodiment of the invention, said polymeric material comprises at least one water-soluble polymer and at least one water-insoluble polymer.
A blend of water-soluble and water-insoluble polymers may have a blending ratio in the range from 1:5 to 5:1 and preferably equal to 1:1.
In processes according to the invention, 3-20% by weight, preferably 3-10% by weight, particularly preferably 3-6% by weight, of at least one polymer are dissolved in an organic solvent and electrospun into a porous fiber. The fibers of the invention have diameters from 20 to 1500 nm, preferably 20 to 1000, particularly preferably 20 to 500, most preferably 20 to 100, nm.
The volatile organic solvent used may be dimethyl ether, dichloromethane, chloroform, ethylene glycol dimethyl ether, ethylglycol isopropyl ether, ethyl acetate or acetone or a mixture thereof with or without further solvents. The vaporizing step may be carried out at atmospheric pressure or else under reduced pressure. If necessary, the pressure shall be adapted to the boiling points of the solvents.
It is advantageous to use solvents or solvent mixtures in the process which are a theta solvent for the polymer/polymer blend in question. The polymer solutions may also pass through the theta state during the electrospinning process. This is the case for example during the vaporizing of the solvent.
For polymer solutions in the theta state see Elias, H. G., in Polymer Handbook, IIIrd Ed., John Wiley and Sons, 1989; section VII.
These solutions are spun by electrospinning. Typically a polymer solution is continually pumped into spinnerets or, in the lab, into a spray cannula whose diameter is not more than 0.5 mm in the case of the apparatus available. The field strengths between cannula and counterelectrode may be 2xc3x97105 V/m and the distance may reach 200 mm. This produced uniform fibers having diameters from 20 to 4000 nm, as can be seen in the scanning electron micrograph of FIG. 1. Instabilities may also lead to irregular thick places on the as-spun filaments. The surprising regular morphology, which is characterized by open pores, becomes apparent in the enlargements of FIGS. 2 to 5. The production of the porous polymeric nano- and mesofilaments is illustrated in the examples.
The porous fibers of the invention have a large surface area of above 100 m2/g, preferably above 300 m2/g, especially above 600 m2/g, and most preferably above 700 m2/g. These surface areas can be calculated from dimensions derived from scanning electron micrographs or measured by the BET nitrogen adsorption method.
The porous fibers produced by the process of the invention can be processed into wovens, drawn-loop knits and shaped and also structured pressed stock; wet-chemically and plasma-chemically modified; or loaded with materials having different objectives, for example pharmaceutically active entities or catalytic precursors, by impregnating and subsequent drying.
The porous fibers of the invention may further be used as ad- or absorbents, in the biological sector (biomaterial) and also as templates for producing highly porous solid articles (for example ceramics by casting and burning out the polymeric templates).
The porous fibers of the invention may further be subjected to surface modification using a low temperature plasma or chemical reagents, for example aqueous sodium hydroxide solution, inorganic acids, acyl anhydrides or halides or else, depending on the surface functionality, with silanes, isocyanates, organic acyl halides or anhydrides, alcohols, aldehydes or alkylating chemicals including the corresponding catalysts. Surface modification may be used to confer on the porous fibers a more hydrophilic or hydrophobic surface, and this is advantageous for use in the biological or biomedical sector.
Porous fibers according to the invention can be used as reinforcing composite components in polymeric materials of construction, as filter materials, as carriers for catalysts, for example as a hydrogenation catalyst after coating of the pores with nickel, or for pharmaceutically active agents, as a scaffolding material for cell and tissue cultures and for a wide variety of implants where, for example, osseointegration or vascularization are used structurally. Epithelium cells are thereby readily cultivable on porous polystyrene fibers. It is similarly possible to apply osteoblasts to porous polylactide carriers and to grow a cell tissue by differentiation.
A further surprising effect is the anisotropy of the porous fibers according to the invention, which is identifiable by their birefringence. They are therefore particularly useful as a reinforcing component in fiber composites, where the large internal surface area provides effective bonding and strength for the polymer matrix, especially after suitable surface modification.
In another embodiment of the invention, ternary mixtures of two polymers, of which one is water soluble, for example polyvinylpyrrolidone, polyethylene oxide, polypropylene oxide, polysaccharides or methylcellulose, and a volatile solvent or solvent mixture is spun. These ternary solutions were electrostatically spun in the same manner as the binary mixtures recited above. Nano- and mesofibers were formed, but they did not possess porous morphology. A nonporous structure is obtained for the fiber when conventional electrospinning processes are used. It is advantageous in conventional electrospinning processes to use polymer solvents that are remote from the theta state and do not pass through it during the spinning process.
Only after a water treatment at elevated temperatures, which led to the water-soluble polymer component being dissolved out, did the fiber materials exhibit a porous morphology comprising channel pores extending at least to the fiber core and/or through the fiber; see scanning electron micrographs in FIG. 6.
This fiber material too can be processed into wovens, drawn-loop knits and formed and also structured pressed articles; surficially modified and also functionalized; and be directed to the hereinabove recited uses.